Research by medical sociologist Nicholas Christakis and colleague at the Harvard Medical School in Boston revealed how oher people's happiness, depression, and obesity can affect you:

Recent research shows that our moods are far more strongly influenced by those around us than we tend to think. Not only that, we are also beholden to the moods of friends of friends, and of friends of friends of friends - people three degrees of separation away from us who we have never met, but whose disposition can pass through our social network like a virus.

Indeed, it is becoming clear that a whole range of phenomena are transmitted through networks of friends in ways that are not entirely understood: happiness and depression, obesity, drinking and smoking habits, ill-health, the inclination to turn out and vote in elections, a taste for certain music or food, a preference for online privacy, even the tendency to attempt or think about suicide. They ripple through networks "like pebbles thrown into a pond", says Nicholas Christakis, a medical sociologist at Harvard Medical School in Boston, who has pioneered much of the new work.

At first sight, the idea that we can catch the moods, habits and state of health not only of those around us, but also those we do not even know seems alarming. It implies that rather than being in charge of where we are going in life, we are little more than back-seat drivers, since most social influence operates at a subconscious level.

i actually took a class at harvard with christakis, one of the profs who authored this study. not only was it a great class, he also presented his findings to us in the last unit of the course - the study was completed after some 35-odd years of analyzing weight and weight gain in social networks. he showed us a time-lapse movie that looked kind of like the pictures shown and it was amazing - you could see one large dot representing an obese person connected to a smallers dot representing that person's close relations/friends, and over time the smaller dots would grow in size (dots are BMI i think...) if i can find the video online, i'll post it in another comment!